1. Field of the Invention
This invention relates to a method of manufacturing a coil, in particular a high-voltage coil, having at least one slot portion intended to be arranged in a stator or rotor of an electrical machine.
2. Description of the Prior Art
A coil part which is arranged in slots in a stator or a rotor of an electrical machine normally comprises a bundle of a plurality of conductor lengths arranged adjacent each other and provided with a conductor insulation for electrically insulating the conductor lengths from one another. The bundle may consist of turns of a single electrically insulated conductor. It may also consist of several separate electrically insulated conductors, which are electrically connected to one another, usually parallel-connected.
The bundle of conductor lengths is surrounded by an insulation for insulating the bundle from the machine slot which has ground potential. This insulation usually comprises material which has strong resistance to corona attack, for example mica or silicone rubber, and its application around the bundle of conductor lengths normally involves impregnating and pressing operations so that an insulation is obtained which has no voids and which tightly surrounds the bundle of conductor lengths.
British Patent Specification No. 867,763 describes one method of manufacturing a coil for an electrical machine. In this prior specification the slot portion, i.e. the portion of the coil intended to be fitted in a slot of a stator or rotor of an electrical machine, is given its final shape by using a compressible mould consisting of two L-shaped mould parts and a heat-shrinkable tape arranged around the mould. The mould assembly is heated causing the heat-shrinkable tape to compress the L-shaped mould parts so that these make contact with each other and thus form a closed mould cavity of rectangular cross-section. The insulating material surrounding the bundle of conductor lengths may be impregnated with a thermosetting or a thermoplastic impregnating resin. When using a thermosetting impregnating resin, this is supplied to the insulating material around the bundle of conductor lengths before the insulating material is applied around the conductor bundle, that is, before the conductor bundle is arranged in the mould. After the compression of the mould and after the evacuation of the mould with its contents, the thermosetting impregnating resin is cured with a heated liquid pressure medium, such as molten asphalt, which is arranged around the mould and the heat-shrinkable tape. Since the heat-shrinkable tape is applied as a tight joint, the pressure medium does not penetrate into the insulation. When using a thermoplastic impregnating resin, the insulating material is applied around the bundle of conductor lengths in an unimpregnated state. After compression of the mould in the manner described above, and evacuation thereof, the thermoplastic impregnating resin is in this case arranged around the mould and the shrinkable tape and the impregnation takes place while the thermoplastic impregnating resin is maintained under pressure.
In the prior art method described above, it is the L-shaped mould parts inside the heat-shrinkable tape that give the coil part intended to be fitted in the machine slot its final shape. This means that great demands are placed on the dimensional stability of the mould parts. In particular the mould parts need to be made of thick, relatively expensive material which, in order to be economic, have to be re-used. However, this means that the mould parts have to be subjected to cleaning processes between moulding operations. Since the mould parts in compressed state form a closed mould cavity, the mould offers a considerable resistance to the supply of an impregnating resin to the conductor bundle and its insulation after compression of the mould. The same is true of the tight joint of the heat-shrinkable tape.
The present invention aims to overcome the disadvantages described above.